Written by Dr. David Bushart Edited by Dr. Hayley McLoughlin

A newly-proposed treatment strategy might be effective against several forms of spinocerebellar ataxia and other CAG repeat-associated disorders

Upon receiving an initial diagnosis of spinocerebellar ataxia (SCA), a swarm of questions might enter a patient’s mind. Many of these questions will likely revolve around how to manage and treat their disease. What treatments are currently available to treat SCA? What can I do to reduce symptoms? Does SCA have a cure, and if not, are researchers close to finding one? Patients and family members who read SCASource may be able to answer some of these questions. Although scientists are aware of some of the underlying genetic causes of SCA, and patients can benefit greatly from exercise and physical therapy, there are unfortunately no current drug therapies that can effectively treat these diseases. However, this is a very exciting time in SCA research, since researchers are hard at work developing new treatment strategies for several of the most common SCAs. Many of these newly proposed therapies are specialized to treat a specific genetic subtype of SCA (e.g. SCA1, SCA3, etc.), which would allow these therapies to be very specific. However, these specialized efforts beg another question: would it be possible to treat different types of SCA with the same therapeutic strategy?

This is precisely what researchers wished to determine in a recent study, authored by Eleni Kourkouta and colleagues. This group of researchers used a technology called antisense oligonucleotides (often abbreviated ASO, or AON), to ask whether a single ASO could be used to treat multiple neurological disorders that have different underlying causes. Currently, most ASO technology depends on our ability to selectively target specific disease-causing genes, which allows the ASO to only recognize and act on the specific gene that is causing ataxia. Once recognized, these ASOs can recruit cellular machinery that lowers RNA levels of the disease-causing gene, thereby greatly limiting the amount of disease-causing protein that is produced (learn more in our What is RNA? Snapshot). This strategy has the potential to be very effective for treating SCAs that are associated with polyglutamine (polyQ) expansion (learn more in our What is Gene Therapy? Snapshot).

However, the type of ASO technology described above is not the only way to reduce levels of the disease-causing proteins in SCA. In this paper, Kourkouta and colleagues use a different type of ASO with a different mechanism of action, which also lowers levels of the disease-causing protein in two different SCAs.

The authors have previously developed an ASO known as “(CUG)7”, which was shown to effectively reduce mutant polyQ protein levels in mouse models of Huntington’s Disease, a separate neurological disorder that is genetically very similar to SCA. Interestingly, the design of (CUG)7 does not depend on cellular machinery that degrades RNA to prevent the expression of polyQ proteins, as described above. The authors found that (CUG)7 works by a different mechanism, in which (CUG)7 binds to the specific region of the RNA that contains the polyQ-coding sequence and physically blocks cellular machinery that allows RNA to be translated into protein. Since the disease-causing genes in polyQ SCA contain the exact same genetic code as in Huntington’s Disease, and since this sequence is recognized by (CUG)7, the authors hypothesized that (CUG)7 may also be effective in improving treating mouse models of SCA.

In order to test their hypothesis, the authors investigated two patient-derived cell lines, one from a SCA3 patient and another from a SCA1 patient. These cell lines contain the same polyQ-expanded genes that cause SCA3 and SCA1, respectively, in these patients. As such, the researchers were able to test the cellular effects of (CUG)7 treatment in both SCA3 and SCA1. The authors found that in SCA3 cells, (CUG)7 caused a phenomenon known as “exon skipping”, in which the exon, or protein-coding sequence, of the RNA that contains the expanded polyQ sequence was passed over by cellular protein translation machinery. The authors proposed that exon skipping may limit the production of polyQ SCA3 proteins, an effect that was observed in SCA3 cells. As higher levels of (CUG)7 were administered to these cells, the levels of Ataxin-3 protein became further reduced. Interestingly, a similar exon-skipping phenomenon was observed in SCA1 cells, although at a slightly lower level than in SCA3 cells.

Since (CUG)7 could effectively target the RNA sequences that cause SCA3 and SCA1 in cell lines, the authors wished to determine whether this treatment could be used to effectively reduce the levels of polyQ disease proteins in mouse models of SCA. The authors injected (CUG)7 directly into the ventricles of the brain of SCA3 or SCA1 mice, a delivery strategy that has been successfully used in many previous studies to widely deliver drug throughout the brain. (CUG)7 was infused once per week, for six weeks, before assessing delivery and effectiveness of the compound. In SCA3 mice, (CUG)7 was detected throughout the brain, including in the brainstem and cerebellum, which are two important brain regions to treat in SCA3. The authors also found that delivery of (CUG)7 also corresponded to a reduction in polyQ-expanded Ataxin-3 protein levels in these same brain regions. Similarly, in a mouse model of SCA1, (CUG)7 was also widely detected throughout the brain after the same delivery method and again resulted in reduced levels of polyQ-expanded Ataxin-1 protein. Overall, the authors showed that (CUG)7 may be able to effectively target the production of the polyQ-expanded proteins that cause SCA3 and SCA1, and can be delivered successfully in mouse models of SCA.

The results of this study are encouraging for the development of better tools to treat SCAs that are caused by polyQ expansion. However, it is important to note that there is still much work to be done before (CUG)7 could be used to treat human SCA and other diseases, like Huntington’s Disease. Although (CUG)7 is non-specific and can potentially be used to treat several different diseases, this also means that it has the potential to have off-target effects. In future studies, it will be important for the authors to determine whether (CUG)7 has a preference for expanded polyQ sequences that cause disease, or whether other genes that have a normal CAG repeat range are also targeted and reduced by (CUG)7. In addition, while the authors expect that (CUG)7 will also improve motor function in SCA3 and SCA1 mice, this was not assessed in these studies and will need to be performed before (CUG)7 is further developed as a human therapy.

Overall, Kourkouta and colleagues have designed a new, intriguing therapeutic strategy to treat several different SCAs, which adds to the growing body of evidence that new treatment strategies for SCA are possible. This is an exciting time for research in SCA, and for the development of new therapies that can soon be used to treat patients.

Key Terms

Antisense oligonucleotide (ASO): Anti-sense oligonucleotide. These small molecules bind to RNA and prevent the production of the protein that the RNA produces.

Polyglutamine (polyQ): A type of protein that contains a large stretch of glutamine amino acids, which is genetically encoded as the nucleotide sequence CAG. Several SCAs are caused by expanded glutamine sequences in polyQ proteins.

RNA: Ribonucleic acid. This molecule copies the information encoded in genes (which are made of DNA) and functions as a blueprint for making proteins in a cell.

Translation: The process after transcription where RNA is converted into a protein sequence.

Conflict of Interest Statement

The authors and editor declare no conflict of interest.

Citation of Article Reviewed

Kourkouta E, Weij R, Gonzalez-Barriga A, Mulder M, Verheul R, Bosgra S, Groenendaal B, Puolivali J, Toivanen J, van Deutekom JCT, Datson NA (2019) Suppression of mutant protein expression in SCA3 and SCA1 mice using a CAG repeat-targeting antisense oligonucleotide. Molecular Therapy: Nucleic Acids, Volume 17:601-614. https://doi.org/10.1016/j.omtn.2019.07.004